Bump mapping is used to add detail to an image without increasing the number of polygons. Bump mapping relies on light-reflection calculations to create small bumps on the surface of the object in order to give it texture; the surface of the object is not changed.
Bumps are applied by matching up a series of grayscale pixels with colored pixels on the rendered, colored object. Lighter grayscale pixels create a sense of maximum relief or maximum indentation; darker pixels have less effect.
A computer must contain a supporting 3D graphics card when it runs an application that has been coded to include bump maps. If the graphics card does not support bump mapping, then the bumps won't be seen. In the case of computer games, the programmer usually will code an alternate version that doesn't use bump maps. This version will look flatter and less real.